(1) Field of the Invention
The present invention relates to the field of aircraft, and more specifically rotorcraft. Rotorcrafts are aircrafts presenting the feature of having at least one rotary wing that serves to provide them at least with lift, and possibly also with propulsion and/or with the ability to maneuver in flight. Such a rotary wing is driven in rotation at a generally constant speed by a power plant having one or more engines.
The rotary wing may equally well be a main rotary wing providing at least the lift if not also the propulsion of the rotorcraft and/or its maneuverability in flight, or a propulsive propeller in a hybrid helicopter, for example, or indeed a tail rotary wing serving to enable the rotorcraft to be maneuvered in yaw.
The present invention relates more particularly to a fluid flow circuit for feeding fuel to a said power plant dedicated to driving the rotation of at least one rotary wing of a rotorcraft.
(2) Description of Related Art
Said power plant may have one or more engines, depending on regulation categories that are well known in the field of rotorcraft. In order to limit the number of engines on board a rotorcraft, it is common to use a single power plant to drive the rotation of various rotary wings fitted to the rotorcraft. Conventionally, the power plant is located on the rotorcraft as close as possible to the main rotary wing that is situated on top of a cabin of the rotorcraft, with the engine(s) being fed with fuel via a fluid flow network forming part of the rotorcraft.
Such a fluid flow network has a main fuel tank that is commonly located under the cabin. The main tank may include one or more compartments with natural fluid flow communication between them. For a main tank of large capacity, it is common practice to have a feeder tank housed within the main tank. The fuel is taken from the feeder tank to feed an engine allocated thereto. The feeder tank is formed by an enclosure placed inside the main tank and it presents capacity suitable for enabling the engine to be supplied with a quantity of fuel that is predefined with respect to regulatory conditions for safe flight. The feeder tank and the main tank form capacities that are independent with respect to free communication of fuel from the main tank to the feeder tank.
The feeder tank is supplied with fuel from the main tank by a transfer circuit that uses a transfer pump housed inside the feeder tank. The transfer circuit comprises in particular an ejector or an analogous member placed inside the main tank in order to trap a contribution of fuel and discharge it towards the feeder tank. From a flow of a primary fuel stream generated by the transfer pump inside the transfer circuit, such an ejector induces the capture of a secondary fuel stream that is discharged together with the primary stream out from the ejector into the feeder tank. Since the capacity of the feeder tank is limited in terms of said predetermined quantity of fuel, the feeder tank has an overflow device that allows excess fuel to be restored to the main tank by overflowing.
Given the proximity between the main tank and the feeder tank, and given the low head losses induced by the transfer circuit, the transfer pump is a low pressure pump, by way of indication it operates at a pressure of less than 200 millibars (mbar) approximately, and more commonly less than 100 mbar. The transfer pump is driven in particular by electricity taken from the electricity network on board the rotorcraft.
In general terms, the ejector comprises an upstream nozzle for admitting a primary fluid stream into a main channel that has a constriction. A secondary channel for admitting a secondary fluid stream opens out into the main channel upstream from the nozzle. The flow of the primary stream inside the main channel acts by suction to capture the secondary stream of fluid via the secondary channel. The secondary stream and the primary stream mix together upstream from the constriction and then the total stream of fluid from this mixture is exhausted from the ejector downstream from the constriction. The notions of upstream and downstream should be considered relative to the flow direction of the streams in question through the ejector.
An ejector is a member commonly used in the field of aviation for fluid flow networks for feeding fuel to an engine. Conventionally, an ejector enables the flow rate of fuel flowing through the fluid flow network to be regulated to match requirements. By way of example, this ability of an ejector is used to feed the feeder tank from the main tank via the transfer circuit, as mentioned above. Other applications are known for such ejectors in the context of feeding fuel to an aircraft engine.
For example, according to document U.S. Pat. No. 3,275,061 (Boeing Co.), a feed ejector is located at the bottom of the feeder tank on a circuit for feeding fuel to an engine of an aircraft, in particular an airplane. A suction pump is used to take fuel from the feeder tank and to convey it to the engine via the feed circuit. The suction pump is also used for transferring fuel between the main tank and the feeder tank via a transfer circuit that is added to the fuel circuit and that is provided with a transfer ejector located inside the main tank.
Under such circumstances, the suction pump is a very high pressure pump, conventionally operating at a pressure of about 10 bar. The suction pump takes the quantity of fuel needed for satisfying the fuel requirements of the engine from the feeder tank. Such a quantity of fuel may be the sum of a said primary stream plus a said secondary stream delivered at the outlet of the feed ejector, in order to optimize the potential contribution in fuel to the engine. While the engine is in operation, the feed ejector is used in particular to increase the flow rate of fuel fed to the engine to match its requirements, such as during a stage in which the aircraft is accelerating.
A problem arises for rotorcraft because of the significant distance along the gravity axis between the main tank and the power plant. In the above-mentioned common example of the main tank and the power plant being located respectively under and above the cabin, such a separation distance may, by way of indication, be as much as 2.5 meters (m) or even more.
For safety reasons, the pressure of the fuel conveyed along the cabin of a rotorcraft via a feed circuit connecting a feeder tank to an engine must be limited as much as possible. Conventionally, such a pressure is limited to a pressure of the order of 1 bar. As an indication, the pressure of the fuel conveyed by the feed circuit lies in the range 1 bar to 2 bar. The feed circuit is dimensioned accordingly in order to be capable of delivering the fuel flow rate needed by the engine in its nominal mode of operation.
In a first common method of feeding fuel to a rotorcraft power plant, the feeder tank(s) respectively allocated to an engine house respective booster pumps in their bottoms. Such a booster pump is immersed inside the feeder tank and is driven in particular by electricity taken from the electricity network on board the rotorcraft. The booster pump takes fuel from the bottom of the feeder tank and discharges it through the feed circuit to the engine.
In a second common method of feeding fuel to a rotorcraft power plant, the or each engine is individually fitted with a suction pump that takes fuel by sucking it from the feeder tank allocated to the engine. The fuel is driven by suction through the feed circuit, the suction pump taking the quantity of fuel that is necessary for satisfying the fuel requirements of the engine.
An advantage of that second method lies with respect to rotorcraft safety in the event of the feed circuit being broken, as might happen during an emergency landing, for example. Once the feed circuit is broken, the suction pump is no longer capable of taking fuel from the feeder tank, thus making it possible to avoid fuel escaping and being spread out from the main tank through the broken zone of the feed circuit.
Nevertheless, a problem raised by that second method lies in priming the suction pump in the event of air being present inside the feed circuit. Because of said distance separating the feeder tank from the suction pump that is located close to the power plant, such air might be present, e.g. as a result of the feed circuit being connected to the atmosphere during a maintenance operation on the power plant, or indeed as a result of failed attempts at starting the engine that have been repeated successively.
In order to solve that problem, a conventional solution lies in adding a priming pump to the fluid flow network in order to prime the suction pump. Such a priming pump is placed in the bottom of the feeder tank in fluid flow communication with the feed circuit. The priming pump is in particular powered electrically from the electricity network on board the rotorcraft.
The priming pump is activated deliberately by the pilot of the rotorcraft, prior to starting the engine, in order to supply fuel to the feed circuit and prime the suction pump. Once the engine has started, the priming pump is deactivated and the suction pump provides the fuel flow necessary for the requirements of the engine. The flow of fuel conveyed to the engine via the feed circuit is regulated by the suction pump in response to control means for controlling the injection of the fuel delivered by the suction pump into the engine.
In the field of aviation there is a need to seek to limit the number of members on board an aircraft, but without that harming the operating safety of the aircraft, nor the ability of the aircraft to comply with regulatory safety conditions in the event of malfunction.
There is also a need in the field of aviation to seek to improve the ergonomics of aircraft in order to off-load the pilot as much as possible of tasks to be performed and in order to avoid causes of malfunction that might potentially be induced by pilot negligence.
In the light of these needs and in the context of said second method of feeding fuel to a rotorcraft power plant, it appears that the conditions for priming the suction pump would benefit from being improved.